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Water activity of glucose solutions,

from [2220]

 

Water activity of glucose solutions see ref [2220]

Water Activity

Water activity (aw) is a measure of how easy the water content may be utilized.

 

V Definition of water activity

V The effect of salt
V The effect of temperature
V The control of activity in foodstuffs

 

      'aw has become one of the most important intrinsic properties used for predicting the survival and growth of microorganisms in food '

  Chirife and Fontana, 2007 [2476]

Definition of water activity

The term 'water activity' (aw) describes the (equilibrium) i amount of water available for hydration of materials (i.e. water activity = water availability; the ratio of the thermodynamic activity to the analytical concentration). When it is all available, aw = 1, and when none is available, aw = 0. If water interacts with solutes and surfaces, it is unavailable for other hydration interactions and its effective concentration in the solution is reduced. A water activity value of unity indicates pure water whereas zero indicates the total absence of 'free' water molecules; the addition of solutes always lowering the water activity (see above right [2220]). Unity water activity does not imply the bulk water is unstructured, only that it is indistinguishable from 'pure' water. Water activity has been reviewed in aqueous [788] and biological systems [1813] and has particular relevance in food chemistry and preservation.

 

Water activity (aw) is defined as equal to the ratio of the fugacity (f, the real gas equivalent of an ideal gas's partial pressure; fugacity is the escaping tendency of a substance; and f0 is escaping tendency of pure material) of the water to its fugacity under reference conditions (f0), but it approximates well to the more easily determined ratio of partial pressures (P) under normal working conditions.

aw = f/f0 ~ P/P0  

    

The activity coefficient (γw) has dependence on the partial molar volume and hydrogen bond strength (which includes dependence on the temperature and dielectric constant) of the water and only in dilute solutions (that is, aw > 0.95) can it be approximated by unity. The water activity (aw) is related to the chemical potentialw; at equilibrium, μw of liquid water and its vapor phase are identical) by

μw = μw° + RTLn(aw)

 

where μw° is the standard chemical potential of water. At equilibrium, μw is the same everywhere in the system establishing this equilibrium by movement of water from regions of high water activity to regions of low water activity. Multiplication of water activity by 100 gives the equilibrium relative humidity (ERH) in percent.

 

aw = P/P0  = ERH (%)/100

 

Thus, water activity is easily and accurately determined from equilibrium relative humidity. Water activity is the effective mole fraction of water, defined as aw = γwxw = P/P0 a where γw is the activity coefficient of water, xw is the mole fraction g of water in the aqueous fraction, P is the partial pressure of water above the material and P0 is the partial pressure of pure water at the same temperature (that is, the water activity is equal to the equilibrium relative humidity (ERH), expressed as a fraction). It may be experimentally determined from the dew-point temperature of the atmosphere in equilibrium with the material [473, 788]; for example, by use of a chilled mirror (in a hygrometer) to show the temperature when the air becomes saturated in equilibrium with water. f, h A high aw (that is, > 0.8) indicates a 'moist' or 'wet' system and a low aw (that is, < 0.7) generally indicates a 'dry' system. Water activity reflects a combination of water-solute and water-surface interactions plus capillary forces. The nature of a hydrocolloid or protein polymer network can thus affect the water activity, crosslinking reducing the activity [759]. Note that the water activity of all aqueous solutions in equilibrium with ice (awi) is equal to the water vapor pressure over ice to the water pressure over pure liquid water and does not depend on the solute's nature or concentration [457]. Solutions with the same ice melting point therefore have the same water activity.

 

There appears to be a common water-activity living limit (aw ~ 0.61) for all life forms (Archaea, Bacteria, and Eukarya) [2427].

 

The water activity (aw) is related to the net isosteric heat of sorption (qst) by the following expressions derived from the Clausius-Clapeyron equation,

delta(ln(aw)/delta T=isosteric heat of sorption/RT^2              isosteric heat of sorption/=-R(delta(ln(aw)/delta(1/T)            qst = Qst - ΔHvap

where T is the absolute temperature (K), R is the universal gas constant (kJ ˣ kg-1 ˣ K-1). qst is the net isosteric heat of sorption (kJ ˣ kg-1), Qst is the isosteric heat of sorption (kJ ˣ kg-1) and ΔHvap is the heat of vaporization of water (kJ ˣ kg-1). The net isosteric heat of adsorption is defined as the negative of the differential change in total enthalpy (ΔH°, kJ ˣ kg-1) for a differential change in the surface excess of water at constant system temperature (T). As the binding is strongest for the first water molecules that bind, the heats of sorption increase with decreasing moisture content [3094].

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The effect of salt

Molality - activity for some salts

 

Molality - activity for some salts
Water activity is reduced by the presence of salts (see left). Ideally, the water activity should follow a straight line with molality. Deviations from ideality are best shown using the osmotic coefficient (ϕ).

 

The relationship between the osmotic coefficient and water activity is:

Phi=-LN(water activity)/(Mw x nu x molality)

where Mw is the molar mass of water in kg ˣ mol-1, ν is the number of ions formed when one mole of salt is dissolved in water (in these cases = 2), ms is the salt molality, nw is the moles of water and ns is the moles of salt.

 

The effect of surfaces on the interfacial water activity

 

Cartoon showing the effect of surfaces on the interfacial water activity

 

Surfaces control the water activity of neighboring water pools compared with the activity of bulk water (see right). At hydrophobic surfaces, the water molecules have 'dangling' hydrogen bonds and consequently high water activity. Such water has a tendency to move towards the lower activity bulk. If the surfaces are flexible, they will tend to close up the pore, as happens in protein structural formation and between hydrophobic and nanobubble covered surfaces. If the surface forms strong hydrogen bonds to the interfacial water then this water will have lower water activity than the bulk. As described elsewhwere, this causes the tendency of external water to move inwards and so cause increased osmotic pressure. A consequence of these water flows is the opposite behavior of solutes as shown in the diagram.

 

A solution of high activity water can be made by treatment with gold nanoparticles illuminated by green-light-emitting diodes [2831]). This high-activity water is negatively-charged, has a lifetime of about a week and possesses (from its high activity) the expected higher vapor pressure and lower specific heat.

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The effect of temperature

Water activity versus sorption isotherm

 

Water activity versus sorption isotherm

Shown right is an indicative water activity isotherm displaying the hysteresis often encountered depending on whether the water is being added to the dry material or removed (drying) from the wet material. This hysteresis is due to non-reversible structural changes and non-equilibrium effects. The water activity isotherm is a curve of the equilibrium moisture content versus the water activity. It can be determined by measuring water content when aw is controlled [3093] or by measuring the sample’s aw when the water content is fixed [3092]. These methods have been compared [3091].

 

There are many empirical equations (and tables) that attempt to describe this behavior but, although indicative, none predict with sufficient accuracy and the water activity isotherm should be experimentally determined for each material. In the food industry, such empirical equations combine contributions from the ingredients to give an estimate of aw, which is then used to estimate the mold-free shelf life (MFSL; Log10(MFSL,days)=7.91-(8.1 ˣ αw) , 21 °C, [443]).

 

The water activity (aw) usually increases with temperature and pressure increases. e For small temperature increases (T1 -> T2) at low aw, an often-applicable relationship is:

 Natural log of (the water activity at T2 over water activity at T1)=(enthalpy change/gas constant)x(1/T1 - 1/T2)

 

where ΔH is an enthalpy change (for example, absorption or mixing), R is the gas constant and T is in Kelvin. A similar equation is derived on the colligative properties page. Such changes in water activity may cause water migration between food components. Increasing the temperature reduces the mold-free shelf life.

 

The multi-ingredient nature of food and its processing (for example, cooking) commonly result in a range of water activities being present. Foods containing macroscopic or micro-structural aqueous pools of differing water activity will be prone to time and temperature dependent water migration from areas with high aw to those with low aw; a useful property used in the salting of fish and cheese but in other cases may have disastrous organoleptic consequences. Such changes in water activity may cause water migration between food components. As the humidity of the air is typically 50-80% (aw = 0.5-0.8), foods with lower aw will tend to gain water whilst those with higher aw tend to lose water. [Back to Top to top of page]

The control of activity in foodstuffs

Control of water activity (rather than water content) is very important in the food industry as low water activity prevents microbial growth (increasing shelf life), causes large changes in textural characteristics such as crispness and crunchiness (for example, the sound produced by 'crunching' breakfast cereal disappearing above about aw = 0.65) and changes the rate of chemical reactions (increasing hydrophobe lipophilic reactions but reducing hydrophile aqueous-diffusion-limited reactions), flavor and smell of food. The balance between these factors is such that there is an optimum water activity for dehydrated foods, which is usually equated with a monolayer coverage of water and an aw of about 0.2 - 0.3 [1127]. Highly perishable foodstuffs have aw > 0.95 (equivalent to about 43 % w/w sucrose), Growth of most bacteria is inhibited below about aw = 0.91 (equivalent to about 57 % w/w sucrose); similarly most yeasts cease growing below aw = 0.87 (equivalent to about 65 % w/w sucrose) and most molds cease growing below aw = 0.80 (equivalent to about 73 % w/w sucrose). The absolute limit of microbial growth is about aw = 0.6. b As the solute concentration required to produce aw < 0.96 is high (typically > 1 molal), the solutes (and surface interactions at low water content) will control the structuring of the water within the range where aw knowledge is usefully applied. Changes in the natural clustering of water due to low concentrations of solutes will only occur at aw > 0.98. Although low-density water (ES) will possess less aw than collapsed water clustering (CS) and the consequences are very important in biological systems, such changes in the absolute value of aw are small.

 

Indicative values of water activities
Substance
γw
xw
aw
seawater
1.0
0.98
0.98
Saturated LiCl
0.19
0.57
0.11
Saturated MgCl2
0.83
0.40
0.33
Saturated SrCl2
1.03c
0.69
0.71
Saturated BaCl2
1.18 c
0.76
0.90
Bread
-
35 d
0.96
Cheese
-
37 d
0.97
Dried fruit (for example, sultanas)
-
18 d
0.76
Raw meat
-
60 d
0.98
Dry pasta
-
12 d
0.50
Cooked pasta
-
72 d
0.97
Preserves (for example, jam)
-
28 d
0.88

 

The safe storage of food is also controlled by the pH at lower water activities; thus for example at a water activity of 0.92, only pH's above pH 4.2 present potential microbiological hazards in non-heat-treated food [1127]. [Back to Top to top of page]

 


Footnotes

a  Prediction equations for the water activity of multicomponent systems have been developed [552], based on the Gibbs-Duhem equation Volume x change in pressure - entropy x change in temperature -  sum from i=1 to i=N of the relative proportion of component i times its change in chemical potential, which at constant temperature and pressure simplifies to sum from i=1 to i=N of the relative proportion of component i times its change in chemical potential and therefore sum from i=1 to i=N of the relative proportion of component i times its change in Ln(activity), where the terms ni are the relative proportions of components n of chemical potential μ and activity a. The resultant equations, although starting on this firm theoretical base, require empirical simplifications due to the problems involving the interactions between the components and the paucity in our knowledge of the molecular interactions of the components with water. Water activity prediction may also be achieved by combining the effects of the chemical groups (rather than molecules) present, where suitable parameters are available [557]. In conclusion, prediction of the water activity of mixed components presents difficulty and, except in cases of simple interpolation, is best determined experimentally. [Back]

 

b   Note that the required aw necessary to prevent growth will depend somewhat on the solutes present; for example, glycerol lowers aw efficiently but still may allow microbial growth. [Back]

 

c   An activity coefficient (γ) less than unity for ions may be due to non-ideal behavior caused by the removal of water by binding to the ions (see colligative properties page). An activity coefficient (γ) greater than unity for ions may be due to non-ideal behavior caused by the volume taken up by large ions (and other solutes, for example, sucrose) at high concentrations [442]. An activity coefficient (γw) greater than unity for water may be simply seen as due to the removal of some of the ions as separate solutes by the formation of ion-pairs (see for example, [997]). [Back]

 

d   % w/w. [Back]

 

e   In some materials (for example, salts and some sugars) water activity may reduce with temperature increase. At high pressures, water behaves similarly to solutions with increasing salt content in that the water activity apparently reduces with increased pressure [457]. [Back]

 

f   There is a number of methods for measuring water content [470] including the poorly understood Karl Fischer titration [471]. [Back]

 

g   The mole fraction of water equals the number of moles of water divided by the total number of moles of all materials, including water, in the same volume. [Back]

 

h   The activity coefficients for solutes may be determined in several ways, including boiling point elevation, freezing point depression, equilibrium vapor pressure, equilibrium relative humidity (ERH, aw ~ ERH/100) [2294], osmotic pressure (aw = exp(-ΠVm/RT)), heat of dilution and excess heat capacity [929] and the Raman absorption at 180 cm-1 [1963]. Due to deviations from the theoretical relationships applied, different methods may give different results, particularly at high solute concentrations. [Back]

 

i   The activity coefficient is an equilibrium property. Note that most food (and other biological) materials during preparation and/or processing will not be at equilibrium and so their properties may diverge from those expected from their activity coefficients. Although varying with water content, water activity is a distinct property. Activity values are empirical values determined by experiment. [Back]

 


 

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